Definition of a property of some text, or its paragraph. Defines
how to find out if it's present (via the mask on the paragraph or
character "contains" header field), how long the value of it is,
and how to get and set the value.
As the exact form of these (such as mask value, size of data
block etc) is different for StyleTextProps and
TxMasterTextProps, the definitions of the standard
TextProps is stored in the different record classes